This invention relates to a method for offshore production of liquefied natural gas (LNG), wherein the gas is supplied from an underground reservoir as either associated or non-associated gas. In the case of associated gas, which is produced in association with oil production, there is no way to transport it to market in the absence of a pipeline. This gas has often historically been flared. More recent aspirations to decrease the environmental consequences of producing oil have increasingly led to the gas being re-injected into underground reservoirs. This is costly and not always practical. Liquefaction of this gas offers a way to transport this gas to market by reducing the gas volume in the liquid phase at low temperatures.
Increasingly, liquefaction of natural gas in non-associated stranded gas fields has been considered to allow these stranded resources to be exploited. Offshore liquefaction of natural gas has not yet seen widespread implementation because of a few fundamental limitations. LNG is required to be produced and stored at low temperatures. This introduces a number of challenges.
One of the first challenges to the liquefaction of associated gas is developing a liquefaction process and transportation system that meets the requirements of the gas producers. An associated gas producer's primary interest and revenue streams are often associated with crude oil production. Naturally, this means that their requirements are considerably different than those of either onshore LNG producers or offshore large-scale LNG producers from non-associated gas fields. Prime consideration therefore has to be given to natural gas liquefaction processes which complement the oil production and processing operations in an offshore environment.
However, because the prior art has tended to focus on adapting the existing on-shore concepts to offshore liquefaction, there remain several limitations of the prior art when applied to associated gas processing. The limitations of the prior art when applied to associated gas processing are:                Process availability has been based on onshore LNG schemes that tend to focus on single trains of large compressors that must all be running to produce LNG;        Prioritising process efficiency at the expense of operability by developing dual expander and mixed refrigerant processes adapted to attempt to preserve efficiency expected at large scale onshore LNG plants, which increases the process complexity;        Inherent safety has typically been compromised by hydrocarbon inventories within mixed refrigerant processes but also by the amount of cryogenic processing equipment, and the operator's unfamiliarity with extensive cryogenic processing;        Prior processes have failed to recognise and address the implication that personnel working in crude oil production and processing may not be familiar with cryogenic processes, equipment, or storage.        
Process availability is critical for associated gas producers because an unavailable plant means that either crude oil production is decreased or the gas is flared whilst the plant is down. Adopting on-shore large-scale LNG processes has resulted in minimum redundancy and acceptance of a resultant loss in availability when one of the large compressor sets is down. The present invention seeks to address this limitation of the prior art.
Operability is another limitation of many of the processes developed for offshore liquefaction. As is appreciated by those skilled in the art, operability is generally improved when a process has solid anchor points for robust control, a low equipment count, minimal compositional complexity (including refrigerants) or minimal process recycles.
Many offshore processes are geared towards large scale constant rate gas production profiles. The scale of this production is usually governed by the size of the LNG carrier and LNG storage volumes. The present invention seeks to take account of declining gas production rates typically associated with oil production operations.
Inherent safety is a big driver offshore. Some highly efficient onshore processes derived from mixed refrigerant and dual mixed refrigerant processes offer very good thermodynamic performance but at the cost of decreased inherent safety, increased process complexity, and decreased operability.
The present invention seeks to deliver a robust, simple, and highly available process with a thermodynamic performance and inherent safety levels not available in existing processes.
In recent years, much research has been started to look for a natural gas liquefaction process that is especially suitable for offshore application. Several liquefaction cycles have been proposed for the liquefaction on a Floating Production, Storage and Offloading vessel (FPSO). Reijnen and Runbalk (U.S. Pat. No. 6,658,891B2) from Shell Research Ltd developed a LNG liquefaction process for offshore applications by using a two-phase single mixed refrigerant with a pre-cooling evaporating refrigerant. However, the complexity of the mixed refrigerant greatly influences the offshore operation and safety issues.
A nitrogen refrigeration cycle may be more appropriate for small-scale offshore operations. Although a nitrogen cycle has the disadvantage of lower thermal efficiency, fuel consumption is a less significant cost item in the overall scheme of the whole facility, and so a nitrogen cycle may be advantageous in terms of safety and energy footprint.
Prible et al (U.S. Pat. No. 6,889,522 B2) proposed an LNG production process from offshore stranded gas reserves using dual independent expander refrigeration cycles. Nitrogen and methane are used as refrigerants for the two separate cycles. Fredheim et al. (U.S. Pat. No. 7,386,996 B2) also developed an offshore LNG plant using a carbon dioxide based pre-cooling circuit cascade associating with a nitrogen-rich main cooling circuit. The process efficiency is improved by mixing with the nitrogen small amounts of hydrocarbons, though the inherent safety is compromised. The cascade arrangement of the process limits the LNG process in large scale applications.
Dubar et al. from BHP Petroleum PTY Ltd (U.S. Pat. No. 6,250,244 B1) proposed an offshore liquefaction apparatus using a dual expander cycle for the gas phase refrigerant which is typically nitrogen. The split of nitrogen refrigerant reasonably distributes the cold energy required in the different temperature ranges resulting in better fitting of the cooling curve. The process is suitable for offshore small scale LNG production from stranded gas.
However, all the above offshore LNG production processes focus on relatively large scale (>1 MTPA) processes and the feed gas is mainly stranded gas from the gas field. However, little attention is paid to the rich associated gas which serves as the feed gas of an offshore LNG plant. The increasing use of offshore oil production apparatus (e.g. FPSO) makes the associated gas widely obtainable. Due to the heavy hydrocarbon containing properties of the associated gas, natural gas liquids (NGL) extraction is necessary before the feed gas enters the main liquefaction heat exchanger to avoid the freezing of heavy components at cryogenic temperatures, thus a need for good heat integration exists to keep both the NGL extraction and the LNG process efficient and economical.